17 Professional Land Surveyors of Oregon | www.plso.org THE EPIC SURVEY OF MASON AND DIXON Two other attempts were too far west. As the task seemed beyond the capability of the local surveyors, the Penns and the Calverts consulted the Astronomer Royal, who recommended Charles Mason and Jeremiah Dixon. Dixon was an experienced surveyor from County Durham, England, and Mason had been an assistant at the Royal Observatory in Greenwich. Th ey had worked together on the Transit of Venus observation of 1761, an international scientifi c eff ort to determine the distance from the earth to the sun and the size of the solar system— burning questions of the 18th century. Mason and Dixon entered into a contract with the proprietors and arrived in Philadelphia on November 15, 1763, to begin work. Th ey brought with them two state-of-the-art instruments specially commissioned by the Penns for the task. Th e fi rst was a zenith sector, the most advanced instrument for determining latitude of its day. It had a six-foot telescope mounted over a protractor scale used to determine latitude by measuring the angles of reference stars from the zenith in the sky. Th ey also brought a transit and equal altitude instrument that determined true north by tracking stars where they crossed the meridian. Mason and Dixon also used other instruments, including a Hadley Quadrant, an octant used in celestial navigation that can measure angles up to 90 degrees, and an astronomical regulator, a pendulum clock in a tall case. Th e men began their historic task at the southernmost point of Philadelphia, the north wall of a house on Cedar Street (now under the bed of I-95). From complex astronomical observations, Mason and Dixon determined their latitude to be 39°56'29.1" N. Th is became the reference for the east-west line, which would become the Maryland-Pennsylvania border, and was 15 miles due south from their starting point. Because going the required 15 miles due south to start the east-west line would have taken them across the Delaware River and through the Province of New Jersey, the surveyors decided to proceed west 31 miles, to the farm of John Harland in what is now Embreeville, Pennsylvania, or as they put it, in the “forks of the Brandywine.” In the spring of 1764, they set off exactly 15 miles due south to a point where they set an oak post, which they called the “Post Mark’d West in Mr. Bryan’s fi eld” near what is now Newark, Delaware. Th is was to become the starting point of the famous west line and the reference point for the rest of the survey. As it was used as a base for the calculations that followed and is mentioned almost daily in their journal, it is the most signifi cant point in the survey. » continues on page 18 » » THE EPIC SURVEY OF MASON AND DIXON, from page 16
PLSO Issue 1 2015 Jan_Feb
To see the actual publication please follow the link above